Improve Dfg type system (#6390)

Added a mini type system for Dfg using DfgDataType to replace Dfg's use
of AstNodeDType. This is much more restricted and represents only the
types Dfg can handle in a canonical form. This will be needed when
adding more support for unpacked arrays and maybe unpacked structs one
day.

Also added an internal type checker for DfgGraphs which encodes all the
assumptions the code makes about type relationships in the graph. Run
this in a few places with --debug-check. Fix resulting fallout.

src/V3DfgDfgToAst.cpp

@@ -257,8 +257,9 @@ class DfgToAstVisitor final : DfgVisitor {
     DfgToAstVisitor(DfgGraph& dfg, V3DfgDfgToAstContext& ctx)
         : m_modp{dfg.modulep()}
         , m_ctx{ctx} {
-        // Convert the graph back to combinational assignments
+        if (v3Global.opt.debugCheck()) V3DfgPasses::typeCheck(dfg);
 
+        // Convert the graph back to combinational assignments
         // The graph must have been regularized, so we only need to render assignments
         for (DfgVertexVar& vtx : dfg.varVertices()) {
             // If there is no driver (this vertex is an input to the graph), then nothing to do.